1. Field of the Invention
This invention relates to radio frequency automatic direction finding systems and, particularly, to an automatic direction finding system utilizing amplitude modulation impressed on the received radio frequency signal. It concerns circular antenna array systems that continuously observe a complete 360 degrees in azimuth to instantly determine the direction of an incoming signal. This is a narrow aperture type direction finding system in which the diameter of the antenna is always less than one half wavelength.
2. Description of the Prior Art
Radio direction finding has long been used by aircraft and marine service to determine the location of the receiving station as an aid for navigation. The ADF systems which are commercially available usually consist of a sense antenna, a loop antenna, and a combiner/phase shifter to form a special pattern. This type of ADF usually operates over a narrow band of one octave or less and is limited to a number of discrete frequencies with a special dedicated receiver.
Other methods use mechanical rotation of loops or other directional antennas. Mechanical systems, besides being bulky and unsightly, require a large amount of power and have all of the problems associated with physically rotating an antenna. Still other systems derive the bearing information by use of the null, rather than lobe because the null is narrow and will provide more accuracy when adequate signal strength is available. The primary disadvantage of the null type DF or the rotating directional antenna is that the signal is always fading in and out destroying the intelligence. Another problem with conventional automatic direction finding systems utilizing the null is that under weak signal conditions the signal strength approaches the sensitivity threshold of the receiver, thereby broadening the width of the null and limiting the system accuracy. Furthermore the time required to obtain a bearing is on the order of several seconds creating a particular problem with unlicensed transmitters because the signal may not stay on very long.
Automatic direction finding systems which utilize the doppler principle produce excessive switching noise on the signal. Low switching rates in or below the audio range generally produce significant problems in detection of the transmitted intelligence. As the switching rate is increased, the system bandwidth must be increased correspondingly, thereby sacrificing system selectivity. However, even in the more advanced systems where the chopping rate is well above audio, the switching transients still disrupt the intelligence on the signal. Additionally the requirement of an omnidirectional pattern for each of the antennas in a doppler array usually limits the practical bandwidth because the gain of the antennas falls off very rapidly with decreasing frequency.